Comey made his comments before the House Judiciary Committee in Washington, where he said the FBI was initiating the investigation in Phoenix but will "follow wherever the facts take us."

The VA has confirmed whistle-blowers' allegations that at least 18 Arizona veterans died while awaiting care in the Phoenix VA Health Care System and that patient-wait times were manipulated for staff to receive financial bonuses.

The inspector general said in a report last month that 1,700 veterans seeking treatment at the Phoenix VA hospital were "at risk of being lost or forgotten." The VA has confirmed that at least 35 veterans died while awaiting treatment in Phoenix although officials say they do not know whether the deaths were related to long waiting times for appointments.

An audit of VA facilities nationwide confirmed similar allegations in other states. The scandal has prompted the resignations of two top VA officials, and termination actions have been against several administrators including the former Phoenix VA director.

The Senate passed legislation Wednesday to allow veterans to seek care outside the VA system and allocate money to hire more VA medical staff. The House passed a similar measure Tuesday that also would ban all VA staff bonuses through 2016.

The VA's Office of Inspector General previously had indicated that its investigators found evidence of actions that could constitute criminal activity and that it had requested assistance from the U.S. Department of Justice. While it is unclear what role the FBI is playing - Comey would not elaborate on his remarks - recent IG reports indicated that VA administrative actions in Phoenix required deeper investigation.

Comey said the inquiry began in Phoenix because that is where the original allegations were made.

He did not elaborate on the investigation, but a Justice Department official said Wednesday that the department had formally asked the FBI to investigate allegations related to the VA. The official, who said the FBI would review materials provided by the inspector general's office, spoke on condition of anonymity because the official was not authorized to discuss an ongoing criminal investigation on the record.

Perryn Collier, a spokesman for the Phoenix FBI office, did not respond to requests for comment.

Sen. John McCain, R-Ariz., was among numerous members of Congress to call for a criminal investigation of the VA.

Richard Griffin, the VA's acting inspector general, issued a scathing report last month that confirmed allegations of excessive waiting time at VA hospitals and inappropriate scheduling practices. He recently told lawmakers his investigators were probing for wrongdoing at 69 agency medical facilities, up from 42 two weeks earlier.

The VA, which serves almost 9 million veterans, has been reeling from mounting evidence that workers falsified reports on wait times for medical appointments in an effort to mask frequent, long delays. An internal audit released this week showed that more than 57,000 new applicants for care have had to wait at least three months for initial appointments and an additional 64,000 newly enrolled vets who requested appointments never got them.

VA Secretary Eric Shinseki resigned May 30, but the situation remains a continuing embarrassment for Obama and a potential political liability for congressional Democrats seeking re-election in November.